In the manufacture of extremely fine detailed structures, a workspace is typically required that has a low level of environmental pollutants, such as dust, microbes or chemical vapors. For instance, in the fabrication of certain semiconductor devices, optical components and pharmaceutical products, the presence of particulates as small as one micron can cause considerable defects, since the size of the product that is being produced may be comparable, or even significantly smaller, in size.
To minimize the risk of particulate contamination, sensitive products are commonly manufactured within a pollutant controlled system. As defined herein, the term “pollutant controlled system” encompasses various types of enclosed constructs that are designed to create a clean, low particulate workspace and includes, but is not limited to, (i) smaller, bench-mountable hoods, each typically constructed as a multi-sided enclosure with an enlarged front opening, (ii) larger inflatable canopies, or tents, each typically constructed with a free-standing frame covered with fabric walls, and (iii) entire rooms, or cleanrooms, within a manufacturing facility into which multiple technicians can comfortably enter and perform requisite tasks.
Pollutant controlled systems of the type as described above are typically classified in efficiency by measuring the number of certain sized particles per cubic foot of air. A particle counter is a light-scattering instrument that is commonly used to measure the concentration of airborne particles at designated sampling locations within a tested workspace.
For perspective, an ordinary room in an untreated manufacturing facility can contain at least as many as 1,000,000 particles of at least 0.5 micron size per cubic meter of air. A workspace with this particle count is commonly referred to in the art as a Class 1,000,000, or ISO 9, standard workspace.
By contrast, pollutant controlled workspaces can often achieve considerably lower particle counts. For instance, a Class 1,000, or ISO 6, standard workspace contains no greater than 1,000 particles of at least 0.5 micron size per cubic meter of air. Additionally, a Class 100, or ISO 5, standard workspace contains no greater than 100 particles of at least 0.5 micron size per cubic meter of air. Further, a Class 1, or ISO 3, standard workspace contains no greater than a single particle of at least 0.5 micron size per cubic meter of air. As can be appreciated, maintaining a fabrication workspace with limited particle counts as set forth above has been found to considerably improve product yields.
Regardless of its size and/or construct, pollutant controlled systems often rely upon a downward, laminar flow or air that is produced by at least one fan filter unit disposed directly above the intended workspace (e.g. in the ceiling of a cleanroom). For instance, referring now to FIG. 1, there is shown a simplified section view of a conventional pollutant controlled system 11 that maintains a low particulate workspace 13 using a downward, laminar flow of clean air.
As can be seen, system 11 includes a cabinet-like enclosure 15 comprising a plurality of walls, or panels, 17 that together define clean workspace 13. A fan filter unit 19 is mounted in the top of enclosure 15 and includes a fan, or blower, 21 disposed above at least one filter 23 that is designed to remove at least 99.97% of particles greater than 0.3 micron in size.
In use, blower 21 continuously drives source air 25 through filter 23 to yield clean, or treated, air 27. As part of the design of fan filter unit 19, clean air 27 is blown vertically downward through workspace 13 as a disperse laminar flow. This laminar air flow pushes any airborne particles 29 present within workspace 13 vertically downward. The trapped, contaminated air is then either treated or extracted entirely from workspace 13 through a vent, or outlet, 31 located in a lower region of enclosure 15.
As can be appreciated, the larger the size of the designated clean workspace, the greater the volume of clean air that must be routinely treated and cycled therethrough. For this reason, it has been found to be considerably less expensive to create and maintain clean workspaces of smaller volumes. Accordingly, it is often desirable to customize an enclosure for a pollutant controlled system that is slightly larger than the particle sensitive process or machine for which it is intended.
Pollutant controlled systems that rely upon a downward, laminar air flow have been found to suffer from a couple notable shortcomings.
As a first shortcoming, pollutant controlled systems that rely upon a downward, laminar flow have been found to be ineffective when the workspace is populated with one or more horizontally disposed elements (e.g. tables, shelves or sections of a continuous flexible substrate, or web, driven horizontally by rollers through different treatment stations). Specifically, downward displacement of debris across horizontal elements creates circulation issues within the workspace. As a result, areas of poor, or stagnant, air circulation are created immediately beneath each horizontal element and, as such, produce regions in which particulates accumulate. Additionally, the presence of horizontal elements causes turbulence in the clean air flow, resulting in the pressurization and swirling of particles throughout the workspace. Accordingly, particulates present throughout the workspace can eventually fall by gravity or be attracted electrostatically onto potentially sensitive surfaces, resulting in less than optimal production levels.
As a second shortcoming, pollutant controlled systems that rely upon a downward, laminar flow have been found to be ineffective when the enclosure is opened (e.g. through a door or window) in order to access the pollutant controlled workspace. Most notably, opening the enclosure not only causes clean air to prematurely escape from the workspace but also contaminated air to enter into the workspace, thereby creating inefficiencies.